Carbon steel is steel in which the main interstitial alloying constituent is carbon in the range
of 0.12–2.0%. The American Iron and Steel Institute defines carbon steel as the following:
"Steel is considered to be carbon steel when no minimum content is specified or required
for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium,
or any other element to be added to obtain a desired alloying effect; when the specified
minimum for copper does not exceed 0.40 percent; or when the maximum content specified for
any of the following elements does not exceed the percentages noted: manganese 1.65, silicon
0.60, copper 0.60." The term "carbon steel" may also be used in
reference to steel which is not stainless steel; in this use carbon steel may include
alloy steels. As the carbon percentage content rises, steel
has the ability to become harder and stronger through heat treating; however it becomes
less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability.
In carbon steels, the higher carbon content lowers the melting point.
Types
Carbon steel is broken down into four classes based on carbon content:
Mild and low-carbon steel Mild steel, also known as plain-carbon steel,
is the most common form of steel because its price is relatively low while it provides
material properties that are acceptable for many applications, more so than iron. Low-carbon
steel contains approximately 0.05–0.320% carbon making it malleable and ductile. Mild
steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness
can be increased through carburizing. It is often used when large quantities of
steel are needed, for example as structural steel. The density of mild steel is approximately
7.85 g/cm3 and the Young's modulus is 210 GPa. Low-carbon steels suffer from yield-point
runout where the material has two yield points. The first yield point is higher than the second
and the yield drops dramatically after the upper yield point. If a low-carbon steel is
only stressed to some point between the upper and lower yield point then the surface may
develop Lüder bands. Low-carbon steels contain less carbon than other steels and are easier
to cold-form, making them easier to handle. Higher carbon steels
Carbon steels which can successfully undergo heat-treatment have a carbon content in the
range of 0.30–1.70% by weight. Trace impurities of various other elements can have a significant
effect on the quality of the resulting steel. Trace amounts of sulfur in particular make
the steel red-short, that is, brittle and crumbly at working temperatures. Low-alloy
carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1,426–1,538 °C.
Manganese is often added to improve the hardenability of low-carbon steels. These additions turn
the material into a low-alloy steel by some definitions, but AISI's definition of carbon
steel allows up to 1.65% manganese by weight. Low carbon steel
<0.3% carbon content, see above. Medium carbon steel
Approximately 0.30–0.59% carbon content. Balances ductility and strength and has good
wear resistance; used for large parts, forging and automotive components.
High-carbon steel Approximately 0.6–0.99% carbon content.
Very strong, used for springs and high-strength wires.
Ultra-high-carbon steel Approximately 1.0–2.0% carbon content. Steels
that can be tempered to great hardness. Used for special purposes like knives, axles or
punches. Most steels with more than 1.2% carbon content are made using powder metallurgy.
Note that steel with a carbon content above 2.14% is considered cast iron.
Heat treatment
The purpose of heat treating carbon steel is to change the mechanical properties of
steel, usually ductility, hardness, yield strength, or impact resistance. Note that
the electrical and thermal conductivity are only slightly altered. As with most strengthening
techniques for steel, Young's modulus is unaffected. All treatments of steel trade ductility for
increased strength and vice versa. Iron has a higher solubility for carbon in the austenite
phase; therefore all heat treatments, except spheroidizing and process annealing, start
by heating the steel to a temperature at which the austenitic phase can exist. The steel
is then quenched at a high rate causing cementite to precipitate and finally the remaining pure
iron to solidify. The rate at which the steel is cooled through the eutectoid temperature
affects the rate at which carbon diffuses out of austenite and forms cementite. Generally
speaking, cooling swiftly will leave iron carbide finely dispersed and produce a fine
grained pearlite and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid
steel results in a lamellar-pearlitic structure of iron carbide layers with α-ferrite between.
If it is hypereutectoid steel then the structure is full pearlite with small grains of cementite
scattered throughout. The relative amounts of constituents are found using the lever
rule. The following is a list of the types of heat treatments possible:
Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for
over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically
increases, as this is a diffusion-controlled process. The result is a structure of rods
or spheres of cementite within primary structure. The purpose is to soften higher carbon steels
and allow more formability. This is the softest and most ductile form of steel. The image
to the right shows where spheroidizing usually occurs.
Full annealing: Carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for
1 hour; this ensures all the ferrite transforms into austenite. The steel must then be cooled
slowly, in the realm of 20°C per hour. Usually it is just furnace cooled, where the furnace
is turned off with the steel still inside. This results in a coarse pearlitic structure,
which means the "bands" of pearlite are thick. Fully annealed steel is soft and ductile,
with no internal stresses, which is often necessary for cost-effective forming. Only
spheroidized steel is softer and more ductile. Process annealing: A process used to relieve
stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually
heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C.
The image rightward shows the area where process annealing occurs.
Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper
critical temperature and this temperature is maintained for a time and then the temperature
is brought down below lower critical temperature and is again maintained. Then finally it is
cooled at room temperature. This method rids any temperature gradient.
Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this
ensures the steel completely transforms to austenite. The steel is then air-cooled, which
is a cooling rate of approximately 38 °C per minute. This results in a fine pearlitic
structure, and a more-uniform structure. Normalized steel has a higher strength than annealed
steel; it has a relatively high strength and ductility.
Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures
and then rapidly cooled in water, brine, or oil to the critical temperature. The critical
temperature is dependent on the carbon content, but as a general rule is lower as the carbon
content increases. This results in a martensitic structure; a form of steel that possesses
a super-saturated carbon content in a deformed body-centered cubic crystalline structure,
properly termed body-centered tetragonal, with much internal stress. Thus quenched steel
is extremely hard but brittle, usually too brittle for practical purposes. These internal
stresses cause stress cracks on the surface. Quenched steel is approximately three to four
fold harder than normalized steel. Martempering: Martempering is not actually
a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment
applied after an initial quench of typically in a molten salt bath at a temperature right
above the "martensite start temperature". At this temperature, residual stresses within
the material are relieved and some bainite may be formed from the retained austenite
which did not have time to transform into anything else. In industry, this is a process
used to control the ductility and hardness of a material. With longer marquenching, the
ductility increases with a minimal loss in strength; the steel is held in this solution
until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed
to keep the temperature gradient minimal. Not only does this process reduce internal
stresses and stress cracks, but it also increases the impact resistance.
Quench and tempering: This is the most common heat treatment encountered, because the final
properties can be precisely determined by the temperature and time of the tempering.
Tempering involves reheating quenched steel to a temperature below the eutectoid temperature
then cooling. The elevated temperature allows very small amounts of spheroidite to form,
which restores ductility, but reduces hardness. Actual temperatures and times are carefully
chosen for each composition. Austempering: The austempering process is
the same as martempering, except the steel is held in the molten salt bath through the
bainite transformation temperatures, and then moderately cooled. The resulting bainite steel
has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering
is it can only be used on a few steels, and it requires a special salt bath.
Case hardening
Case hardening processes harden only the exterior of the steel part, creating a hard, wear resistant
skin but preserving a tough and ductile interior. Carbon steels are not very hardenable; therefore
wide pieces cannot be through-hardened. Alloy steels have a better hardenability, so they
can through-harden and do not require case hardening. This property of carbon steel can
be beneficial, because it gives the surface good wear characteristics but leaves the core
tough. Forging temperature of steel
See also Cold working
Hot working Welding
Forging References
Bibliography Degarmo, E. Paul; Black, J T.; Kohser, Ronald
A., Materials and Processes in Manufacturing, Wiley, ISBN 0-471-65653-4.
Oberg, E.; et al., Machinery's Handbook, Industrial Press Inc, ISBN 0-8311-2599-3.
Smith, William F.; Hashemi, Javad, Foundations of Materials Science and Engineering, McGraw-Hill,
ISBN 0-07-295358-6.